Throughout time, it has always been important to know who one's friends are. Nowhere has this been more obvious than in military conflicts, where for centuries, flags, banners, insignia and uniforms have allowed adversaries to distinguish their cohorts from others who might have less friendly intentions. And, in darkness, when those visual means of identification were impossible, the business of using passwords and countersigns achieved the same end.
That system worked for millennia as long as conflicts were more or less face to face and visual identification was possible. But about 50 years ago, just as World War II began, the widespread use of aircraft caused a dramatic and inexorable change. Because threats could now approach with great speed by the time the visual identification was possible, it was often too late to prevent destruction. Also, while battle forces were once drawn up on opposing sides of some geographic line in the past, the new battle zones quickly became chaotic mixtures of friendly and hostile forces with many isolated units operating autonomously, thus making identification of others on the battle field nearly impossible.
Visual means were and still are an important method of discriminating between friends and enemies. But airplanes now fly at night, are extremely fast, fly at high altitude and can attack outside of visual range, thus making additional means of detection vital to survival in combat.
The earliest forms of radar were just emerging during WW II and, although it seemed to offer a solution to the problem, a major drawback soon became evident. The radar could detect incoming aircraft at considerable distance by sending out powerful pulses of radio energy and detect the echoes that were sent back, but it could not tell what kind of aircraft had been spotted or to whom they belonged.
The first recorded attempt at an electronic Interrogator Friend or Foe (“IFF”) system was used by the Germans during WW II. By rolling their planes over when they received a predetermined signal, the Germans pilots would change the polarization of the radar reflections picked up by their own ground radars. The pilots created a distinctive blip on the radars that differed from others so the radar operators could identify their friendly forces. This simple yet effective attempt to distinguish their planes from enemy planes to German radar operators incorporated the basic structure of all cooperative IFF systems that followed: a challenge or question (the coded radio message) and a specific response (the roll over that caused a change in the reflected radar signal). This maneuver was a passive system in that the returned signal was still just a reflection of the radar energy sent from the ground.
The first active system employed radio energy generated on the target aircraft which was then used as the return signal identifying the aircraft. This is the basic method now used in all modern cooperative IFF systems. The Mk I was an active IFF system and was put into service in about 1940. It used a receiver aboard each aircraft that broke into oscillation and acted as a transmitter when it received a radar signal. Because of the variety of radar frequencies used, it had to be mechanically tuned across the radar bands in order to be triggered by any radar that was illuminating it. This mechanical tuning requirement and other factors limited its performance. The Mk II was a later improvement of the Mk I, it had a separate transmitter that was tuned through the radar bands simultaneously with the receiver and was triggered by signals from the receiver. This greatly increased the strength of the return signal and the return range. It also could be programmed to respond in one of six different codes thus providing some further degree of identification. The MK II was triggered in the radar bands while the MK III operated in the VHF band.
After the war, with rapid technical developments creating new high performance aircraft, the need for efficient and reliable IFF systems led to a long series of further refinements that eventually evolved into the modern IFF systems in use today. Modern IFF systems are basically Question/Answer systems. An interrogator system sends out a coded radio signal that asks any number of queries. The interrogator system is frequently associated with a primary radar installation, but it may also be installed aboard a ship or another aircraft. The interrogation code or challenge, as it is called, is received by an electronic system known as a transponder that is aboard the target aircraft. If the transponder receives the proper electronic code from an interrogator, it automatically transmits the requested identification back to the interrogating system as a coded radio signal. Because it was developed as an adjunct to the primary echo-type detection radar and is usually used in conjunction with a primary radar, the IFF system is also known as secondary radar.
As noted above, the military purpose of IFF is identification of friendly aircraft by the use of identification codes on the transmission and reply. In this case, called IFF ‘Interrogation’, an aircraft can be requested by either a ground station, land/sea vehicle or another aircraft fitted with an Interrogator to reply to a coded request for identification. Only friendly aircraft who know the code of the day can provide the correct response to the Interrogation. On most military aircraft the IFF Interrogator, which requires extra ‘black boxes’ and antennas to those of the IFF Transponder, is often enacted by mounting the IFF Interrogator antennas on the main radar dish. In this way, when an aircraft is detected by that radar, it can be selectively interrogated by the directional radar dish, without broadcasting the interrogating aircraft's position through the omnidirectional transmissions typical of Transponder systems.
Modern IFF systems use two channels. Typically 1030 Megahertz is used for interrogating signals and 1090 Megahertz is used for reply signals, but other frequencies may be used as IFF systems change. The IFF system begins when an IFF interrogator attempts to exchange information with its target by establishing an uplink to the transponder. IFF interrogation may be between two aircraft, between a ground based radar station and an aircraft, a tank and an aircraft or a missile system and an aircraft. The transponder accepts the interrogator's request for an uplink by signaling back to the interrogator and thus creating a downlink. The link is established between the interrogator and a transponder over a small azimuth angular extent of the main lobe of the antenna in the direction the antenna is pointing. The antenna used to establish the links between the interrogator and the transponder have other lobes besides the main lobe including but not limited to side lobes and rear lobes, if a link is established in more than one lobe this will create a “ghost” image making it appear as if there are two or more transponders rather than one.
To preclude links from being established in the side lobes, Sidelobe Suppression techniques are implemented for both the uplink and the downlink. For example, Interrogator Side Lobe Suppression (ISLS) and Receiver Side Lobe Suppression (RSLS) have been used for this purpose. Some IFF Interrogation installations, such as those on aircraft, require the vertical size of the IFF antenna to be severely limited. Small vertical aperture IFF antennas will typically have a strong back lobe due to a strong forward to rear RF coupling. The backlobe problem can also be compounded by aircraft installation effects including the platform body and the radome. In systems that use a difference beam to cover the sum beam for Interrogator Side Lobe Suppression (ISLS) and Receiver Side Lobe Suppression (RSLS), a mirror image of the IFF forward radiation patterns appearing 180 degrees out the back of the IFF antenna at lower power but still at a sufficient level to cause the interrogator to try to establish a link. ISLS and RSLS are typically ineffective in the rear hemisphere.
The problem with the mirror image appearing in the rear hemisphere is that in addition to a “ghost” being created on the IFF display, which may confuse the pilot, each target is processed twice, once when the antenna is pointed at the transponder and again when the mirror image pattern is pointed at the transponder. This cuts the target processing capability of the system in half. Also, if every transponder has to respond twice to every interrogator, it may not be available to respond to the real interrogations creating an aircraft awareness deficiency in today's high air traffic density.
The common technique to correct for the back lobe of an antenna is to use a backfill radiator. A backfill radiator is simply a smaller antenna located at the rear of the primary IFF antenna. Transmit power is coupled off the difference port and applied to the backfill radiator at a level sufficient to cover the back lobe present when transmit power is applied to the IFF antennas Sum port. This causes the Difference pattern to cover the Sum pattern radiation pattern out the rear of the antenna and restores the effectiveness of the ISLS and RSLS to 180 degrees out from the intended IFF link.
Therefore historically the use of the backfill radiator restores ISLS and RSLS effectiveness at the mirror angle of the main lobe, breaking the inadvertent IFF link at that angle thus preventing the IFF system from wasting computing resources attempting to link with a “ghost” signal. However, using the standard technique with small vertical aperture IFF antenna's typically used on airborne platforms, nulls can be generated in the difference pattern at other angles than 180 degrees in the rear hemisphere where the link can be re-established generating “ghosts”. In cases where the front to back side lobe coupling is tight, the use of the backfill radiator generates nulls in the difference pattern at wide angles in the forward hemisphere, establishing the link at other angles than the mainlobe or mirror angle of the mainlobe. Again, this generates “ghosts”. A backfill therefore eliminates the “ghost” at the mirror angle, but “ghosts” may now be generated at other angles. It therefore is not 100% effective. An example of a region outside the main lobe includes but is not limited to the side lobe and the back lobe.